1. Field of the Invention
The present invention relates to the field of liquid crystal displaying, and in particular to a structure of a pixel that increases a charging speed of a pixel unit.
2. The Related Arts
A thin-film transistor liquid crystal display (TFT LCD) is a display device that has various advantages, including thin device body, saving power, and being free of radiation, and is thus widely used. Most of the liquid crystal displays that are currently available in the market are backlighting liquid crystal displays, which comprise a liquid crystal panel and a backlight module. The liquid crystal panel comprises two glass substrates that are opposite to and parallel with each other and liquid crystal molecules arranged between the two substrates and the operation principle is that a driving voltage is applied to the two glass substrates to control rotation directions of the liquid crystal molecules in order to refract out light emitting from the backlight module for generating images.
Liquid crystal molecules are anisotropic in refractive index. Through application of a driving voltage to the liquid crystal molecules to re-define an anisotropic axis of refractive index, the brightness of the light transmitting through the liquid crystal molecules can be controlled. Since thee are impurity molecules contained in the liquid crystal molecules, polarization may result after long term application of DC voltage driving. Consequently, AC voltage driving is often adopted for liquid crystal displaying. The largest driving voltage that is needed for AC voltage driving is twice of the liquid crystal driving voltage so that an extended period time is needed for charging of a pixel unit.
The formula for the time period necessary for charging a pixel unit is t=RC*ln[(V1−V0)/(V1−Vt)], wherein R stands for the sum of the resistance of a data line and the source/drain contact resistance, C is the sum of coupling capacitance of wiring, V1 is the voltage value that a storage capacitor Cst can be finally charged or discharged (approximately 2*Vcom, where Vcom is reference voltage), Vt is the voltage value of the storage capacitor Cst (which is greater than 0 and less than 2*Vcom), and V0 is the initial voltage value of the storage capacitor Cst.
A conventional pixel unit needs to have the storage capacitor Cst charged so as to have the voltage raised from V0 to Vt within the scanning time of one line. For liquid crystal panels having low resolutions and low refresh rates, the voltage of the storage capacitor Cst can reach a predetermined gray level voltage within the scanning time of one line. However, for a present-day liquid crystal panel having a high resolution and a high refresh rate, it is generally difficult to have a pixel unit charged to a predetermined grey level voltage within the scanning time of one line. Further, with the progress of the liquid crystal displaying technology and people's demand for high quality visual enjoyment, high resolution and refresh of image with high frame speed are now the direction of development of the liquid crystal panels. However, the charging time that a pixel unit needs for high resolution and high frame speed displaying has reached the limit thereof.